Add transport driver for http

[rslemon]

In order to support sending a processed message to http endpoint(e.g: Fluent Bit HTTP input plugin), adding transport driver for http.

Here is example:

$ ./goflow2 -transport=http \
  -transport.http.server=localhost \
  -transport.http.port=4739 \
  -format=json \
  -transport.http.tag=tagname

[rslemon]

@lspgn Could you please review this PR? Please let me know if there is something that needs to be changed. Thank you.

[lspgn]

Hello @rslemon,
Thank you for your contribution but unfortunately I cannot accept this work.
The readme suggests submitting a feature request first. I rejected a similar PR for an http transport and detailed some of the reasons here: #278 (comment)

Two comments:

• This would not be scalable as there is no batching.
• "path" is a preferable term to "tag" but you could pass everything as a single variable called "endpoint".

I would suggest using the standard input to Vector or Fluentbit and convert it to http or a logging system.